The present invention relates generally to resistance welding and more particularly to a method for welding together at least two pieces of steel employing projection resistance welding.
In resistance welding, two pieces of steel are welded by bringing them together, sandwiching them between a pair of welding electrodes, and applying an electric current through the electrodes at the location where the electrodes contact the two pieces of steel; this heats up the two pieces of steel where they are contacted by the electrodes, causing melting of the steel at that location. When the flow of electric current is stopped, the molten metal solidifies as a weld button or nugget, thereby adhering together the two pieces of steel. The heating which the two pieces of steel undergo as a result of resistance welding is proportional to (1) the electrical resistance of the steel material, (2) the square of the electric current, and (3) the time period during which the current flows.
In projection resistance welding, the electric current is more fully concentrated at the location where the two steel pieces are contacted by the electrodes. This is accomplished by pre-forming, on a portion of one of the steel pieces, a series of aligned projections. The aligned series of projections may be continuous, e.g. in the form of a continuous elongated ridge, or discontinuous, e.g. in the form of a plurality of spaced-apart dimples. The two pieces of steel to be welded together are arranged so that a portion of one piece is in superimposed relation with the series of projections on the other piece.
The welding electrodes in projection resistance welding are typically in the form of a pair of rotatable wheels. The two superimposed portions of steel are sandwiched between the pair of rotatable welding wheels. The welding operation then moves either (a) the two pieces of steel relative to the welding wheels or (b) the pair of welding wheels relative to the two pieces of steel. The zone of welding is, at any given time, concentrated at the location where the welding wheels engage the two pieces of steel. Moving either the two pieces of steel or the pair of welding wheels advances the welding zone. The welding wheels rotate as the welding zone is advanced, and pressure is applied to the sandwiched portions of the two pieces of steel by applying pressure through the welding wheels as the latter rotate; this causes a flattening of that part of the series of aligned projections which are sandwiched between the welding wheels in the welding zone. A continuous welding current is applied to the superimposed portions of the two pieces of steel, through the welding wheels, to concentrate the current at those projections which are in the welding zone. The intent is to form weldments where the projections are flattened.
Projection resistance welding can also employ (i) a single, rotatable welding wheel as an upper electrode and (ii) an elongated, stationary copper backing bar, extending the length of the steel pieces undergoing welding, as the lower electrode.
For productivity purposes, it is desirable that the speed of advancement at the welding zone be relatively high. In manufacturing lines where there are manufacturing operations either upstream and/or downstream of the welding operation, it is desirable that the speed of advancement at the welding zone be equal to the speed at which other manufacturing operations are performed either upstream or downstream of the welding operation.
It is also important that the welding current range be relatively wide. A welding current range has minimum and maximum limits. The minimum limit for a welding current range is that current at which welding would be obtained for all welds; this includes not only the welds formed on all projections on a given set of two pieces of steel, but also on all such sets in a continuous manufacturing operation. The maximum limit on the welding current range is that current at which melting occurs on the outer surface of a piece of steel or molten metal is expelled from the interface between the two pieces of steel at the location of a weldment (expulsion).
It is desirable to have a relatively wide welding current range because there are a number of uncontrollable factors which can cause wide fluctuations in the welding current during a given welding operation. Examples of such factors include variations in the surface condition of one or both of the two pieces of steel undergoing welding. Surface conditions include surface roughness and/or rust. Intermittent rust on the surface of a steel piece or variations in rust from a steel piece in one set to a steel piece in another set, during a continuous manufacturing operation, can cause a current change as high as 1,000-2,000 amperes.
The steel pieces are typically formed from steel strip which has been wound into coils which are unwound to form the steel pieces which undergo welding. In a continuous manufacturing operation, several coils may be consumed in a run over a relatively short period of time. Changes in surface conditions or in resistivity from one coil to another can cause variations in current similar to those described in the preceding paragraph.
Other causes of variations in the welding current include electrical malfunctions, power line fluctuations and the like. Indeed, even in well maintained welding equipment, there can be a 400 ampere variation in current, for example. Current changes could also be caused by changes in the pressure exerted by the welding wheels, due to mechanical factors in the pressure mechanism.
Because of all the variables which can affect the welding current, it is desirable to have a welding current range of at least 3,000 amperes.
In order to utilize projection resistance welding in high speed manufacturing operations, the speed of advancement of the welding zone (welding speed) should be substantially greater than about 200 inches per minute (508 cm/min.) e.g. greater than about 250 in./min. (635 cm/min.).
Welding can be enhanced by concentrating the current at the welding zone, rather than dissipating the current around that zone. Concentrating the current, i.e. increasing the current density (current per unit area), has been obtained in the past by narrowing the thickness or width of a welding wheel at its circumference, but this has a drawback in that it can cause undesirable surface marks due to deformation of the surface of the steel engaged by the narrower welding wheel.
Welding wheels having a flat, relatively wide circumferential surface or a convex circumferential surface with a relatively large radius of curvature, avoid the drawback described in the preceding sentence, but lack the current-concentrating properties of a narrower welding wheel.